iPhone 5s, 5c, and the new iPads that were made for mainland China and after September 2013 could not enable the FaceTime Audio in the settings. The function is still blocked if users update the system, use a different SIM card, or change their location to another country.

The problem has been reported on Apple forums since September last year but seems to have remained under the radar. According to the report, consumers not only could not find the audio function on their crippled devices, but they couldn't be audio-called by a non-China model. An error message will pop up every time they receive such calls.

However, FaceTime video on these devices works OK with full audio functions.

It is suspected that Apple cut the audio feature to please the major Chinese carriers-China Mobile, China Unicom, and China Telecom-with the aim to expand further in the lucrative Chinese market, where voice call competition between carriers and apps developers has been fueled by popular messaging apps such as WeChat, according to the report.